In the three cases described above – the case of the object being located beyond 2F, the case of the object being located at 2F, and the case of the object being located between 2F and F – light rays are converging to a point after refracting through the lens. In such cases, a real image is formed. As discussed previously, a real image is formed whenever refracted light passes through the image location.
While diverging lenses always produce virtual images, converging lenses are capable of producing both real and virtual images. As shown above, real images are produced when the object is located a distance greater than one focal length from the lens. A virtual image is formed if the object is located less than one focal length from the converging lens. To see why this is so, a ray diagram can be used.
A ray diagram for the case in which the object is located in front of the focal point is shown in the diagram at the right. Observe that in this case the light rays diverge after refracting through the lens. When refracted rays diverge, a virtual image is formed. The image location can be found by tracing all light rays backwards until they intersect. For every observer, the refracted rays would seem to be diverging from this point; thus, the point of intersection of the extended refracted rays is the image point. Since light does not actually pass through this point, the image is referred to as a virtual image
. Observe that when the object in located in front of the focal point of the converging lens, its image is an upright and enlarged image that is located on the object’s side of the lens. In fact, one generalization that can be made about all virtual images produced by lenses (both converging and diverging) is that they are always upright and always located on the object’s side of the lens.